This invention is particularly directed to improvements in photovoltaic devices and the like and particularly such devices which include at least one semiconductor layer which is a thin polycrystalline film of suitable large grain size. One such semiconductor material is zinc phosphide [Zn.sub.3 P.sub.2 ].
Zinc phosphide is an important semiconductor for photovoltaic solar cells. See, for example, a report by applicant et al, which was published in the Proceedings of the Fourteenth IEEE Photovoltaic Specialists Conference pp. 641-646 [1980] and references cited therein. It possesses a steep optical absorption edge near 1.5 eV where the maximum performance of terrestrially based cells is expected, has a sufficiently long [.about.10 .mu.m] minority carrier diffusion length to permit high current collection efficiency and is comprised of elements which are abundant, thus permitting widespread use. To date, conversion efficiencies as high as 6.08% [total area, 100 mW/cm.sup.2 ELH simulation] have been reported for metal-semiconductor junctions. The efficiency of these cells is limited by a maximum open circuit voltage of 0.5 volts developed by the metal-semiconductor junction. The open circuit voltage of such cells is limited by the .about.0.8 eV barrier height observed for several metals, regardless of work function. Improved open circuit voltage is expected from p/n homojunction cells wherein the junction is formed by contiguous regions of the same semiconductor having opposite conductivity types respectively. The particular advantage of homojunction cells results from minimization of discontinuity in the energy band structure while the gross composition remains the same. Heretofore, development of a Zn.sub.3 P.sub.2 homojunction cell has been prevented by the fact that only p-type conductivity had been observed for Zn.sub.3 P.sub.2 and attempts to prepare the n-type semiconductor by incorporating In, Ga, Sn or Al as substitutional dopants have yielded only high resistivity p-type material. The electrical conductivity of the undoped semiconductor is apparently controlled by native interstitial defects which suggested that a self-compensation mechanism may have prevented preparing the n-type semiconductor by such methods.
A further disadvantage with such semiconductor materials as zinc phosphide is in the difficulties encountered for selecting a proper substrate which meets the necessary criteria while being compatible with high temperature growth.